1. Technical Field
The present invention relates to a ball joint which is, for example, used in a connection portion of a stabilizer for a vehicle, and to a method of manufacturing a housing therefor.
2. Related Art
FIG. 4 shows a conventional embodiment of this kind of ball joint. The ball joint is constructed such that a spherical head portion 2a formed in an end portion of a ball stud 2 is slidably fitted to a resin ball seat 3 received within a cylindrical housing 4. The ball stud 2 is pivoted relating to the ball seat 3 in a universal manner around the spherical head portion 2a, that is, in such a manner as to freely tilt and rotate around an axis. A rivet-like engagement portion 3b formed in a bottom portion of the ball seat 3 extends through a bottom portion of the housing 4 so as to be engaged therewith, whereby the ball seat 3 is restricted to the housing 4, thereby preventing it from rotating around the axis and coming off from the housing 4. In this case, reference numeral 6 denotes a bevel-like dust cover, and reference numeral 7 denotes a support bar integrally adhered to the housing 4.
In this case, as shown in enlarged FIG. 5, the housing 4 is constituted by a main body portion 4a formed in a closed-end cylindrical shape, and a ring-like flange portion 4b formed in an edge portion of the main body portion 4a, and is integrally formed in accordance with a sheet metal process from a steel plate. Further, the support bar 7 is electric resistance welded at a position a distance c apart from a lower surface of the flange portion 4b. This is because the welding is applied so as to avoid a circular arc surface formed in a cross portion between the lower surface of the flange portion 4b and the outer peripheral surface of the main body portion 4a, and a space for burrs generated in welding is secured. Further, the spherical head portion 2a of the ball stud 2 is pressure inserted by elastically deforming the ball seat 3. In order to prevent a crack from being generated in the bottom portion of the ball seat 3 in pressure insertion thereof, it is necessary to provide a certain degree of thickness in the bottom portion. Accordingly, it is impossible to move the ball stud 2 very close to the bottom portion side of the ball seat 3. For the reason mentioned above, a center O of the spherical head portion 2a of the ball stud 2 is arranged at a position a distance e shifted from a center line 7a of the support bar 7.
Since the difference of distance e exists between the center of the spherical head portion 2a of the ball stud 2 and the center line 7a of the support bar 7, a bending moment of P×e with respect to a test load P generated by a tensile test or a buckling test in an axial direction of the support bar 7 is applied to the support bar 7. Accordingly, since a strength of the support bar 7 cannot be obtained while a strength obtained by multiplying a cross sectional area of the support bar 7 by a material strength is generally secured, there have been cases in which it is necessary to make the support bar 7 thick for the purpose of satisfying a required specification.
Further, a drawing load F of the ball stud is generally expressed by a smaller value among values calculated by the following formula (1) or (2).F=4×N×d3×π/4+μ×F′×η×(l1×π×D)  (1)F=N×π×D×l2  (2)
In this case, parameters in the respective formulas mentioned above are as follows.                d: diameter of the shaft in engagement portion 3b         N: shear resistance of the ball seat 3        μ: coefficient of friction between the housing 4 and the ball seat 3        η: component force conversion efficiency in the outer peripheral direction due to contact angle between the ball stud 2 and the ball seat 3 when a drawing load is applied to the ball stud 2        l1: distance of the linear portion extending from the crossing point between a normal line and the inner peripheral surface toward the flange portion 4b (distance of the flange portion 4b to the end point of the circular arc surface) when the normal line is formed passing through the center O of the spherical head portion 2a of the ball stud 2 on the inner peripheral surface of the housing 4        F′: component serving to provide friction force between an inner peripheral surface of the housing 4 and the ball seat 3 when a drawing load is applied to the ball stud 2        D: diameter of spherical head portion        l2: length of thick portion extending from crossing point between normal line and inner peripheral surface toward axial direction of ball stud 2 when a normal line is formed passing through center O of spherical head portion 2a of the ball stud 2 on inner peripheral surface of ball seat 3.        
In general, the value calculated by the formula (1) is smaller in the drawing load F. In accordance with the formula (1), in order to increase the drawing load F, the spherical head portion may be made large, or the housing may be made longer so as to set the distance l1 of the linear portion of the inner peripheral surface to be large. However, in such a countermeasure, weight and size are increased, and material costs are unnecessarily increased.